A known mobile crane is provided with a crane main-body capable of traveling and a counterweight carrier capable of traveling with the crane main-body. The counterweight carrier is used to have a counterweight mounted on it and increase the stability of the crane main-body to enhance the hoisting performance of the crane. Japanese Unexamined Patent Publication No. H5-208796 shows an example of a mobile crane provided with such a counterweight carrier.
The crane disclosed in Japanese Unexamined Patent Publication No. H5-208796 is provided with a crane main-body having a lower traveling body that is self-propelled in a front-back direction and an upper swing body mounted on the lower traveling body to be capable of swinging. In the crane, the lower traveling body is self-propelled as an operation lever used to run the main body is operated, whereby the crane main-body is caused to travel. A counterweight carrier is coupled to the back part of the upper swing body of the crane main-body via a coupling member.
The counterweight carrier is provided with a plurality of wheels and a carrier running motor. The carrier running motor rotates the wheels as the operation lever is operated. Thus, the counterweight carrier is caused to travel with the crane main-body. In addition, each of the wheels is able to swivel about a vertical axis. A traveling direction of the counterweight carrier may be changed by changing an orientation of each of the wheels.
At the traveling of the mobile crane, each of the wheels of the counterweight carrier is steered such that an orientation of each of the wheels corresponds to the front-back direction of the lower traveling body according to a swing state of the upper swing body. However, there is a case that such steering of the wheels requires a long time. The reason for it is as follows.
A rotation direction of the wheels driven by a carrier running motor is made correspondent to each operation of the operation lever by which the lower traveling body is instructed to move forward or backward. In steering the wheels, the wheels are steered such that a movement direction of the wheels when rotating in a rotation direction made correspondent to an operation of the operation lever by which the lower traveling body is instructed to move forward corresponds to the front side of the lower traveling body and such that a movement direction of the wheels when rotating in a rotation direction made correspondent to an operation of the operation lever by which the lower traveling body is instructed to move backward corresponds to the back side of the lower traveling body. Therefore, for example, even if an orientation of each of the wheels is initially set to a direction relatively close to the front-back direction of the lower traveling body, it is required to steer the wheels by an amount close to 180° when a movement direction of the wheels according to an operation of the operation lever is nearly opposite to a traveling direction of the lower traveling body according to the operation of the operation lever. The steering of the wheels requires a long time.